The invention relates to a vacuum brake booster comprising a vacuum chamber and a working chamber separated from one other in a pressure-proof manner by a movable wall, as well as a control valve which comprises a housing workingly coupled to the movable wall and containing a first valve seat, which is disposed in said housing and to achieve a pressure difference at the movable wall is capable of controlling the supply of at least atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster coupled to the first valve seat. Vacuum brake boosters of said type have been known for some time and millions of them are being used to boost the actuating forces of a vehicle hydraulic brake system and thereby keep them at a level, which is comfortably low for the driver of a vehicle.
Also known are so-called braking assistants. By said term is usually meant a system, which in an emergency braking situation, given substantially the same actuating force, may provide a driver with increased braking power. Systems of said type were developed because research revealed that most vehicle users in an emergency braking situation do not press on the brake pedal as hard as is needed to achieve the maximum braking power. The stopping distance of the vehicle is therefore longer than necessary. Systems of said type, which are already in production, use an electromagnetically actuable brake booster combined with a device which is capable of determining the actuating speed of the brake pedal. When said device detects an actuating speed above a preset threshold value, it is assumed that an emergency braking situation exists and the brake booster is set by the electromagnetic actuating device to full output, i.e. it provides its maximum boosting power.
Brake boosters with an electromagnetic actuating facility are however too expensive for low- to medium-priced motor vehicles.
There is therefore a call for solutions which achieve a braking assistance function with a lower outlay.
The object of the invention is to provide a vacuum brake booster of the described type with a braking assistance function without having to have recourse to an electromagnetically actuable control valve. At the same time, unintentional tripping of the braking assistance function is to be avoided as far as possible.
According to the invention, therefore, a permanent magnet and an armature cooperating therewith are disposed in the housing of the control valve. The armature is rigidly coupled, on the one hand, to the input element of the brake booster and/or of the control valve and, on the other hand, to the first valve seat. The armature, which is reciprocable relative to the permanent magnet, is preloaded resiliently counter to actuating direction and in the starting position of the control valve is held at a first distance from the permanent magnet, preferably by means of the resilient preloading counter to actuating direction. In the course of an approach to the permanent magnet, which occurs when the brake booster is actuated, the armature may become less than a previously defined second distance away from the permanent magnet, which is smaller than the first distance. When the armature is less than said distance away, the armature is pulled by the permanent magnet counter to the resilient preloading force acting upon the armature and with simultaneous cancellation of its rigid coupling in actuating direction to the input element into contact with the permanent magnet. Thus, even if the actuating force introduced into the brake booster does not increase, the first valve seat coupled to the armature remains fully open so that the brake booster builds up the maximum pressure difference between its vacuum chamber and its working chamber, i.e. the brake booster provides its maximum boosting power.
In preferred embodiments of the vacuum brake booster according to the invention, said second distance is defined by the magnitude of the resilient preloading force acting upon the armature. When the resilient preloading force acting upon the armature is low, this means that the said second distance is relatively great, i.e. the threshold which has to be crossed to trip the braking assistance function is relatively low. The opposite applies when the resilient preloading force acting upon the armature is high. Thus, by suitably selecting the resilient preloading force acting upon the armature and the said first distance it is possible to preselect a tripping threshold, which on the one hand prevents unintentional full brake applications but on the other hand may still be reliably crossed when necessary, even by drivers who possess less strength.
To enable the previously described tripping threshold to be selected according to requirements without excessively varying the forces to be summoned up by the user during a braking operation, according to a preferred embodiment of the vacuum brake booster according to the invention an element, which is movable relative to the armature, is resiliently preloaded relative to the armature in such a way that it projects from the armature in the direction of the permanent magnet and in the starting position of the control valve (neutral position) is not in contact with the permanent magnet. It is only when the element, which is movable relative to the armature, upon actuation of the vacuum brake booster has been applied against the permanent magnet that the resilient preloading force, which defines the said second distance, acts upon the armature. When the armature is of a hollow-cylindrical design, the element movable relative to the armature and projecting from the latter in the direction of the permanent magnet may, for example, take the form of a spacer sleeve disposed concentrically with the armature. Such a spacer sleeve may be situated radially outside of, or alternatively radially inside, the armature. However, the element movable relative to the armature need not be sleeve-shaped and may alternatively be of a different configuration, e.g. pin-shaped.
In preferred embodiments of the vacuum brake booster according to the invention, the permanent magnet is guided displaceably in the control valve housing and is preferably moreover preloaded counter to actuating direction resiliently against a stop. Such an embodiment has the advantage that, when the armature in the course of a rapidly effected actuation of the brake booster has become less than the previously defined second distance away from the permanent magnet and has consequently been pulled into contact with the permanent magnet and the actuating force exerted upon the input element is further increased, said actuating force is not transmitted from the input element via the unit comprising armature and permanent magnet to the control valve housing and from there to a master cylinder disposed downstream of the vacuum brake booster, rather an increased actuating force exerted upon the input element is transmitted directly from the input element to the downstream master cylinder. The magnetic device, in particular the armature, the permanent magnet and the components accommodating said parts, may therefore be of a less stable design. When, for all conceivable actuating situations, the intention is to prevent the magnetic device from changing into a force-transmitting state, then the permanent magnet has to be displaceable in actuating direction relative to the control valve housing by a distance which is greater than the maximum possible actuating stroke of the input element minus the said first distance.
Given the provision of a stop for the permanent magnet, in order to prevent a misalignment of the contact face of the permanent magnet with the contact face of the armature from arising as a result of manufacturing tolerances and possibly leading to canting and subsequent jamming of the armature and/or permanent magnet, in preferred embodiments the stop for the permanent magnet and/or the contact face, which is formed on the permanent magnet (or the latter""s mounting) and cooperates with the previously described stop, is crowned. Furthermore, there is a radial play between the permanent magnet or the latter""s mounting and the control valve housing. Thus, the permanent magnet may always be correctly aligned with the armature. Alternatively, the armature itself may be fashioned in such a way that its contact face allows a specific swivelling to compensate the said misalignment.
Although the permanent magnet may be resiliently preloaded against the stop in any manner, it is preferred when an, on the whole, annular spring element comprising a plurality of resilient segments of a circle is used to preload the permanent magnet counter to actuating direction. Such a spring element has a very low axial length and therefore reduces the overall length of the control valve equipped with a magnetic device according to the invention.
The stop, against which the permanent magnet resiliently preloaded counter to actuating direction is supported, may be, for example, a step in the control valve housing. Preferably, however, said stop is formed by a stop ring, which is supported in the control valve housing and the axial extension of which defines the said first distance. By virtue of selecting stop rings of differing axial extension it is therefore possible to adjust the said first distance between the armature and the permanent magnet. It is particularly preferred when such a stop ring is made of plastics material in order to deaden the impact noise of the permanent magnet.
In another preferred embodiment of the vacuum brake booster according to the invention, the armature and the permanent magnet are coupled together into a displaceable unit by means of a fettering component, which allows a convergence of the armature with the permanent magnet. In said embodiment the resilient preloading force, which defines the said second distance, acts upon the armature only when the displaceable unit of armature and permanent magnet is resting against a stop, which prevents the displacement of the unit in actuating direction. With such an embodiment the said second distance may be set relatively small without the then necessary, high resilient preloading force acting upon the armature making itself adversely felt in the form of a correspondingly increased response force upon actuation of the brake booster. The displaceable unit of armature and permanent magnet may initially be displaced in actuating direction counter to only a low spring force, during which the position of the armature relative to the permanent magnet does not alter. It is only when the displaceable unit is resting against the said stop that the full spring force acting upon the armature is perceptible to the user of the brake booster. Because such a state, i.e. the resting of the displaceable unit against the stop, occurs only in an emergency braking situation where the driver in any case presses relatively hard and quickly on the brake pedal, the then increased actuating force of the brake booster is in practice not noticed at all by the driver.
The displaceable unit of armature and permanent magnet is preferably guided in the housing of a magnetic device, which housing is fixed in the control valve housing and provides the stop.
Embodiments of the vacuum brake booster according to the invention, which comprise a displaceable unit of armature and permanent magnet, are advantageously designed in such a way that a first spring pushes the armature and the permanent magnet apart from one another and that a second spring preloads the displaceable unit counter to actuating direction. Preferably the joint force of both springs defines the said second distance, below which the force of attraction exerted by the permanent magnet upon the armature exceeds the spring force acting in the opposite direction, with the result that the armature moves into contact with the permanent magnet. In a constructionally advantageous manner, in such embodiments the first spring is clamped axially between the armature and the fettering component and the permanent magnet is fixed relative to the fettering component. The latter may be achieved, for example, by the permanent magnet being accommodated in the fettering component.
In the previously described embodiments with the displaceable unit of armature and permanent magnet, the second spring is preferably supported by its one end against the stop and by its other end against the armature. To save space, the second spring is preferably disposed coaxially with the first spring. When both springs are helical compression springs, the second spring, for example, may be surrounded by the first spring.
According to a preferred refinement of the vacuum brake booster according to the invention with a displaceable unit of armature and permanent magnet, the armature comprises a base and a magnetic plate, which is connected to the latter and faces the permanent magnet. Thus, only the magnetic plate need be made of a material which is attracted by the permanent magnet, while the base of the armature may be made of non-magnetic material, e.g. a plastics material. If the armature is hollow-cylindrical, then the magnetic plate is preferably annular.
In preferred embodiments of the vacuum brake booster according to the invention, the first valve seat of the control valve controlling the ventilation of the working chamber is rigidly coupled in actuating direction via the armature to the input element. The first valve seat may however alternatively be actuated directly by the input element.
In all embodiments of the vacuum brake booster according to the invention, the first valve seat is preferably formed on an, in particular, sleeve-shaped extension rigidly connected to the armature. Thus, each movement of the armature is transmitted without play to the first valve seat.
When the first valve seat is formed on an extension rigidly connected to the armature, a locking bar rigidly connected to the input element preferably extends into a recess of the extension, in which the locking bar in the direction of displacement of the input element has a play which is smaller than the maximum possible actuating stroke of the input element. Thus, the locking bar rigidly coupled to the input element may, during the return movement of the input element, i.e. upon release of the brake, detach the extension rigidly connected to the armature optionally from the permanent magnet, i.e. switch off the braking assistance function. The precise shape of the recess in the extension is in said case unimportant, the only crucial aspect being that, while the return movement is still occurring, the locking bar or some other part rigidly connected to the input element comes into positive engagement with the extension.
In all embodiments of the vacuum brake booster according to the invention, the input element is preferably resiliently preloaded counter to actuating direction. Upon release of the brake, said resilient preloading force returns the input element into the starting position. In a constructionally advantageous manner, said resilient preloading of the input element during its return movement into the starting position is also utilized to detach the armature from the permanent magnet, e.g. by means of the previously described locking bar, which engages into a recess of the extension coupled to the armature.
To reduce or eliminate potentially irritating impact noises which may arise as a result of the return stroke movement of the armature effected after detachment of the armature from the permanent magnet, in all embodiments of the vacuum brake booster according to the invention said return stroke movement is preferably damped by an elastomeric element, which is disposed between the armature and the stop face, towards which the armature moves during its return stroke movement. In a simple and yet effective embodiment, the elastomeric element is an O-ring.
Mostly an emergency braking operation is initiated from the non-operated position of the vacuum brake booster. In other words, the brake system of the vehicle is normally not in operation when an emergency braking situation occurs and the driver will only actuate the brake system in response to the emergency braking situation. However, it is also perfectly possible for situations to arise where, in the course of a braking operation, an emergency braking operation becomes necessary, e.g. when a driver has underestimated how far away an obstacle is. Tripping the braking assistance function in such situations, i.e. when the driver, having already initiated a braking operation, would suddenly like to carry out an emergency braking operation, requires far more work (force x displacement) than tripping the braking assistance function from the non-operated state of the brake system and/or vacuum brake booster. The reason for this is that in the operated state of the brake system the pressures generated in its hydraulic system retroact upon the vacuum brake booster and in particular upon the latter""s input element. During a braking operation, therefore, the pressure prevailing in the hydraulic chamber of the master cylinder disposed functionally downstream of the vacuum brake booster first has to be overcome before the input element of the vacuum brake booster may be displaced further in actuating direction. occasionally, in such situations, drivers possessing less strength may find it difficult to trip the braking assistance function.
In order to guarantee reliable tripping of the braking assistance function also when the brake system is already in operation, i.e. during a braking operation, in preferred embodiments of the vacuum brake booster according to the invention, in which the permanent magnet is displaceable either together with the armature or individually in the control valve housing, the said first distance between the armature and the permanent magnet is reduced in dependence upon the counterforce retroacting from the master cylinder upon the vacuum brake booster. As the counterforce increases, therefore, the first distance between the armature and the permanent magnet is reduced so that the additional displacement needed to reach the previously defined second distancexe2x80x94which substantially defines the tripping threshold of the braking assistance functionxe2x80x94is likewise reduced. Given a suitable design of the system, it is thereby possible to achieve the effect whereby always the same work is needed to trip the braking assistance function, regardless of whether the brake system is in operation or not.
In preferred embodiments, the counterforce acts upon an adjusting sleeve, which is connected to the permanent magnet or its mounting. Said adjusting sleeve may be rigidly connected to the permanent magnet and may moreover comprise a stop, which cooperates with the control valve housing and limits the reduction of the first distance, preferably to a dimension slightly greater than the second distance. Thus, unintentional tripping of the braking assistance function, such as would occur were the first distance to be reduced to less than the second distance, is avoided.
The adjusting sleeve may however also comprise a stop, against which the permanent magnet (or its mounting) is resiliently preloaded counter to actuating direction. Given such a refinement, the permanent magnet may be displaced in actuating direction relative to the adjusting sleeve. This may be advantageous in the tripped state of the braking assistance function in order to prevent a frictional engagement between the permanent magnet and the adjusting sleeve.
Many of the vacuum brake boosters currently in use have at their power output side a reaction disk made of elastomeric material, which is inserted between the vacuum brake booster and the master cylinder connected thereto. As is known to experts in the present field, such a reaction disk improves the response of a vacuum brake booster from its non-operated position. According to an embodiment of the vacuum brake booster according to the invention, the counterforce retroacting from the master cylinder upon the vacuum brake booster is transmitted by said reaction disk to the adjusting sleeve. The adjusting sleeve in turn is supported counter to actuating direction, i.e. in the direction of the counterforce, via a resiliently compliant element against the control valve housing. The compressing of the resiliently compliant element as the counterforce increases then leads to a displacement of the adjusting sleeve counter to actuating direction and hence to the desired convergence of the permanent magnet with the armature. The resiliently compliant element, via which the adjusting sleeve is supported against the control valve housing, may be e.g. an elastomeric ring but alternatively a helical spring, a resiliently preloading disk, a corrugated disk or the like.
According to another embodiment, in which the previously mentioned reaction disk of elastomeric material is not provided, the counterforce retroacting from the master cylinder upon the vacuum brake booster is transmitted by a reaction sleeve preferably directly to the adjusting sleeve so that the latter in turn, as the counterforce increases, is displaced counter to actuating direction. In said embodiment, the adjusting sleeve is preloaded in actuating direction by means of a separate spring.
In the previously described embodiments of the vacuum brake booster according to the invention, even in the inoperative state of the latter magnetic forces still act upon the armature. The springs provided in the control valve are designed so as to counteract said magnetic forces exerted upon the armature in the neutral position by the permanent magnet in order to prevent a convergence of the armature with the permanent magnet. The spring force of the said springs therefore has to be slightly higher than usual, which entails increased actuating forces because the force of the springs has to be surmounted upon actuation of the vacuum brake booster.
To achieve an improvement in said respect, in preferred embodiments of the vacuum brake booster according to the invention a lever arrangement acting upon the permanent magnet is provided, by means of which the said first distance between the armature and the permanent magnet is reduced, upon actuation of the control valve, without falling below the said second distance between the armature and the permanent magnet. Thus, the first distance, i.e. the distance between the permanent magnet and the armature in their neutral position, may be set relatively great without leading to an increase of the displacement needed to trip the braking assistance function. The greater first distance between the permanent magnet and the armature reduces the magnetic forces acting in the neutral position upon the armature, with the result that the appropriate springs of the control valve may be weaker, thereby reducing the force needed to actuate the control valve. As soon as the control valve is actuated, the lever arrangement ensures that the permanent magnet is moved in the direction of the armature in order to reduce the first distance.
The permanent magnet is preferably preloaded resiliently into contact with the lever arrangement so that each movement of the lever arrangement is transmitted directly to the permanent magnet.
In order, in the operated state of the control valve, to achieve a defined distance between the armature and the permanent magnet which naturally has to be greater than the said second distance, upon actuation of the control valve the lever arrangement presses the permanent magnet preferably against a stop formed e.g. at the inside of the control valve housing.
In a particularly preferred embodiment of the vacuum brake booster according to the invention, the lever arrangement is supported by its one lever end against the input element or a power output tappet connected thereto and by its other lever end against the permanent magnet. The control valve housing in said embodiment forms a fulcrum for the lever arrangement.
In a manner which is constructionally advantageous because it is inexpensive, space-saving and operationally very reliable, the lever arrangement is an, on the whole, annular element formed by a plurality of radially extending arms, which are in each case U-shaped in cross section and are connected to one another radially at the inside. The element is centrally penetrated by the power output tappet and supported against a step of the latter. Each arm of the element roughly in the middle of its extension is in contact with a step, which is formed in the control valve housing and therefore forms a fulcrum, about which each arm tilts upon displacement of the power output tappet.
There now follows a detailed description of several preferred embodiments of a vacuum brake booster according to the invention with reference to the accompanying diagrammatic drawings.